A fun little puzzle

I was debating putting this up in General Physics, but since I already know the answer, I thought I'd put it here. I also put this in some other places to see how different forums would react - my own little experiment.
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A true story:
I once took a car GPS with me on an airplane.

It worked! It was fun watching the little box frantically recalculating routes and travel times while the plane happily ignored roads and obstacles. (I turned that part off after a while.) But near the end of the trip, something strange happened.

I started using the GPS about 20 minutes into the flight, and at first, it told me that the plane was moving at about 550 mph. As time passed, the plane's speed slowly and steadily increased - through 600, 700, and as the plane started descending, it reached it's peak of about 790 mph. The speed of sound is just 767 mph, and this ordinary commuter passenger plane was exceeding it! At first I thought that the GPS must be wrong, but then I realised it was quite accurate.

How this was possible? What was really going on?

BTW: The GPS was able to correctly state my altitude. (Or at least was giving me a number that seemed reasonable.)Edit: Oops, I see there is a brain teasers section. Can this be moved?

The airplane entered the jet stream such that it had a very significant tailwind. Its true airspeed, relative to the atmosphere, was nothing atypical. But its airspeed relative to the ground increased significantly due to the addition of the tailwind speed.